INVOLVEMENT OF CAPSAICIN-SENSITIVE PRIMARY AFFERENT-FIBERS IN REGULATION OF JEJUNAL ALANINE ABSORPTION

Capsaicin-sensitive primary afferent fibers (CSPA) in the small intest ine regulate many functions through the release of peptides and neurot ransmitters. This study was undertaken to assess the role of CSPA in t he regulation of jejunal alanine absorption in the rat. In a series of in vivo experiments, the effects of the sensory neurotoxin capsaicin on small intestinal alanine absorption were evaluated. In vitro experi ments were also done to study its effects on alanine uptake by isolate d jejunal strips and mucosal scrapings. Jejunal alanine absorption was reduced by 27% when capsaicin (160 and 800 mu M) was perfused intralu minally and by 21% when it was applied topically to the cervical vagi. On the other hand, bilateral cervical vagotomy and reversible block o f vagal CSPA increased alanine absorption by 29 and 41%, respectively. In vitro, capsaicin reduced alanine uptake by intestinal strips in a dose-dependent manner. Maximal inhibition (36.5%) occurred at 400 mu M with the mean ineffective concentration at 87 mu M. Alanine uptake by jejunal mucosal scrapings, however, was decreased only by 6.7% when i ncubated with 1,600 mu M capsaicin. These data suggest that vagal CSPA exerts a tonic inhibitory effect on alanine absorption and that capsa icin's inhibitory effect on alanine absorption is mediated largely by the capsaicin-sensitive afferent fibers.